Ignore letterbox in two tests

Two tests in EnterLegacySplitScreen were failing from time to time with letter box layer not showing 2+ times. We are ignoring it also in the assertions and refactoring the assertion to only ignore the app layers used in each corresponding case instead of ignoring all of them.

Test: atest WMShellFlickerTests
Bug: b/171049797
Change-Id: I6212f833c790fb0fa002cae2f7a3efc688168578